1. Field of the Invention
The present invention relates to an over-temperature protection method and apparatus thereof for a switching power supply.
2. Description of the Prior Art
At present, most consumer electronic devices adopt switching power supplies as power supplies. The switching power supply controls the energy storage and release of an inductor via switching a power switch to provide a power supply fulfilling specification requirements. In order to prevent damage to equipments and risk to public security, most switching power supplies are equipped with various protection mechanisms, e.g., over-voltage protection (OVP), over-current protection (OCP), over-load protection (OLP), over-temperature protection (OTP), etc. to prevent the occurrence of the aforementioned damage or risk.
An OTP mechanism usually adopts a thermistor whose resistance varies with temperature to observe the temperature variation of a monitored object. When the temperature of the monitored object exceeds a permitted range, at least part of the operation of the switching power supply will be stopped.
FIG. 1 is a diagram of a switching power supply 60 adopting conventional OTP. Switching power supply 60 is a flyback power converter which converts energy inputted by the AC (alternating current) power source VAC into an output power source VOUT which meets a requirement of a specification. Bridge rectifier 62 substantially rectifies the AC power source VAC. Power switch 72 controls a current in primary coil LP in transformer 64. When power switch 64 is turned on, the energy stored in transformer 64 is increased; when power switch 64 is turned off, the energy stored in transformer 64 is released via second coil LS. The released energy is stored in output capacitor 69 through rectifier 66 and therefore generates the output power source VOUT. Feedback circuit 68 monitors a magnitude (e.g., a current, a voltage, or a power) of the output power source VOUT and provides a feedback signal to controller 74.
An OTP mechanism is provided by resistor 78 and thermistor 76, which are connected between an input power source Vin and an electrical ground GND. For example, assuming the resistance of thermistor 76 inversely proportional to the temperature, when thermistor 76 has a higher resistance at a lower temperature, controller 74 receives a logic “0” signal from enable pin “enb” and is thereby enabled. Switching power supply 60 normally provides the output power source VOUT. Once the temperature of thermistor 76 exceeds a certain extent and the resistance thereof becomes smaller, controller 74 receives a logic “1” signal from enable pin “enb”, thereby stopping switching of the power switch 72.